Mass spectra of geometrical isomers: The isomeric 3‐ and 7‐cyclohexyl‐2‐norbornanols

Abstract

Abstract-The mass spectra of isomeric 3-and 7-cyclohexyl-2-norbornanols have been studied. The 3-substituted norbornanols give very similar mass spectra, whereas large differences were observed between the mass spectra of 7-syn and 7-anti isomers. While the mass spectra of these bicyclic alcohols show extensive rearrangements to give rise to many odd-electron ions, the Wagner-Meerwein rearrangement of the molecular ion prior to fragmentation or during the process of dehydration is not important. The origins and the mechanisms for the formation of some major ions are discussed in terms of low voltage spectra, defocused metastables and their relative abundances, and ionization or appearance potentials of the ions of interest.MASS spectra of geometrical isomers are generally similar or identical. In certain cases, however, such isomers can be distinguished by the differences in their mass spectra. As pointed out by Meyerson and Weitkampl in their recent comprehensive review, these isomers generally contain heteroatom(s) which may be involved in bond-making as well as bond-breaking during the process of fragmentation. Such bond-making, e.g. hydrogen transfer to the heteroatom, may be possible with one isomer but not with the other because of the difference in the geometries of the molecules. For the hydrocarbon geometrical isomers, it is still more difficult to differentiate them by their mass spectra even though some differences correlated to the relative enthalpy or 'crowding' of the molecules have been r e p~r t e d .~.~ Several workers have reported the mass spectral fragmentation studies of norbornane derivatives. Most of these workers have expressed the difficulty in distinguishing the differences between the mass spectra of the ex0 and endo isomers> to and also of the syn and anti isomers.8 However, DeJongh, et aL9 reported that the loss of the bromine radical from the 2-norbornyl bromide molecular ion was more facile in the ex0 isomer than the endo isomer. Brion and co-workers,1° using a photoionization source, were also able to observe the differences in the relative intensities of the molecular ions and some major fragment ions in the mass spectra of some isomeric tricyclo [3,2,1 ,O] octane derivatives. These differences were attributed by the authors to non-bonded, steric interactions. This steric or 'crowding' effect was first proposed by Eiemann and Seibl.ll They studied the mass spectra of several epimeric pairs of cyclic secondary and tertiary alcohols, and concluded that the less crowded epimer gave the more intense molecular ion while the more crowded epimer gave the more intense [M -H,O]+. ion.In the present work, we have studied the mass spectra of two sets of geometrical isomers, i.e. isomeric 3-and 7-cyclohexyl-2-norbornanols as shown in Scheme 1. We were concerned with the differences in the mass spectra between the isomers and the origins of the major ions. The low voltage spectra, defocused metastable ions12